Electric drives with a manual doubler feature the capability of the actuator being operated both from an electric motor and by manual control. The necessity of manual control is due, firstly, to operating conditions in the process of adjustment of pipeline sistems and secondly, to possible alarm conditions occuring as a result of the power failure or the failure of electric control system components.
When designing electric drives for controlling pipe valves, special consideration is given to size reduction of the electric drives. This is due to the fact the force exerted by the power transmission is perceived by the gearings used in said electric drives, in operation, as a reaction. Such perception of the force by the gearing requires the size of this gearing to be increased, with the consequent increase in the overall dimensions of the entire electric drive, which are determined by the dimensions of said gearing.
Known in the art is an electric drive with a manual doubler, comprising a frame, a shaft mounted therein, and a pair of threaded elements in engagement, one of them being locked against rotation with respect to the frame, mounted coaxially with the shaft, and capable of axial desplacement, the second element being eccentric to the first one and freely rotatably mounted on the shaft. The two threaded elements combine to form a planetary-helical gearing.
As the threaded element freely supported by the shaft revolves around the stationary threaded element, a slip may occur that results from the frictional moment (force) at the contacting surfaces of the threaded elements being exceeded by the moment (force) of resistance. To avoid this slip, the device is provided with two gear wheels in engagement, one of them being stationary, the other one being located on the eccentric threaded element and adapted to be thrown into engagement with the first wheel. This combination represents a planetary-toothed gearing (SU,A, 992868).
One disadvantage of the above device is large dimensions of the planetary-toothed gearing caused by the reaction of the axial force transmitted being fully sensed by the planetary-toothed gearing. Besides, to ensure an off-line control of the actuator, both from an electric motor and manual, an additional mechanism is required that enables the kinematic chain to be switched over from mechanical to manual control, and vice versa.
Known is an electric drive with a manual doubler, comprising an electric motor, a frame, a sleeve mounted inside the frame, a lead screw, a sliding nut located inside the sleeve, in eccentric relation to the lead screw axis, a gear wheel eccentric to the lead screw axis, connected with the sliding nut, and being in engagement with a gear bush positioned concentrically with the lead screw and rotatably mounted inside the frame, and a manually operated handwheel rigidly coupled to the gear bush (SU, A, 636779).
In this device, the gear wheel is located on the sliding nut, the lead screw and the sliding nut forming, in combination, a planetary-helical gearing, the gear wheel and the gear bush forming a planetary-toothed gearing. The gear bush is locked relative to the frame by means of an adjustable locking device. In case the axial force is in excess of the force the locking device is adjusted to, the locking device enables the gear bush to be turned through in relation to the frame, thus preventing the actuator from being overloaded. The electric drive is disconnected.
One disadvantage of this device resides in the fact that said locking device fixing the gear bush within the frame is adjusted to a specified value of the force that can be only changed in the process of adjusting the device. This force is not connected with the axial force generated in the planetary-helical gearing, as the drive is operated, and fully sensed by the planetary-toothed gearing due to a rigid coupling between the two gearings. Consequently, the maximum force developed in the planetary-helical gearing, which is specified for the electric drive, is also fully sensed by the planetary-toothed gearing, resulting in larger overall dimensions of this gearing and hence, in an increased size of the entire device.
Besides, to ensure an off-line control of the actuator, either from the electric motor or manually, an additional switching mechanism is necessary for the device.